1. Field of the Invention
The invention relates to an improved method for injecting water into fluid storing rocks containing clay minerals such as hydrocarbon sites formed by sandstones, sands and marlitic sandstones, water storing rocks, geothermic circulation sites, and zones in the vicinity of wells extending to large sites.
2. Description of the Prior Art
It is a well-known fact that after primary exploitation of rocks which store hydrocarbons, a considerable amount, about 60% to 75%, of the original (geological) reserves remains unexploited. For the further exploitation of such remaining hydrocarbons, in particular mineral oils, fluid transfer used to be applied from aqueous storing tracts or, in the majority of cases, through properly formed wells, fluid is forced out. Nevertheless, a plurality of secondary and tertiary exploiting methods have been elaborated and up to now simple injection of water has proven to be the most frequently used and most inexpensive solution. From the point of view of economics, such a solution is advantageous. However, success in using such a method cannot always be accomplished since in certain cases difficulties arise because of the characteristics of the water storing rocks. Thus, injecting the water into the rocks may cause problems and the rocks may not adequately retain the water for subsequent injections. In the first phase of injecting water, one is confronted with a problem scarcely dealt with in technical literature, i.e. compatibility of the storing rock and injected aqueous solutions. Thus, injected chemical substances suffer losses in respect to sorption and degradation, and from the moment the the fluid contacts the rock there can be problems with the stability of the rock, state and stability of clay minerals, maintenance of injectability, changes and decline of natural equilibrium, or quasi-equilibrium, and the consequences thereof.
It is also well known that permeability of storing rocks, in particular sandstones containing clay minerals, sands and marlitic sands always depends on the pore structure. However, the presence of mineral clays complicates this simple situation, as their chemical and physical condition depends fundamentally on the ionic environment. Particularly affected are swelling clay materials which change not only their pore structure but also their chemical state in the course of the ion exchange processes which take place in the aqueous phase. The mineral clays of the layer structure 2:1 are capable of swelling, meaning there is an increase in the decrease of cell dimensions in the direction of the C-axis. The change depends on which ions occupy (bind) the free negative charges in the layer structure of clay minerals. If considerable expansion of the clay minerals results in their dispersion, it is quite obvious that hydrodynamic properties of the rock will be negatively influenced.
By analysing the problems from this aspect, it can be stated that, apart from some gas injecting processes, e.g. ethane, gas rich in ethane, etc, in nearly all cases control of sorption phenomena and elimination of detrimental hydrodynamic effects resulting from clay mineral swelling and structural collapse ought to be eliminated. It is well known that we cannot deal merely with ion-exchange, sorption, dispersion or electrostatic dispersion, since these processes take place simultaneously and their advantageous modification may result in diverse effects. As a consequence, if we speak about the changes and modifications of clay minerals, it seems to be expedient to speak uniformly about a clay-effect. We can also refer to decrease, control (exclusion) of its activity inhibition.
It is quite obvious that in the cases of all fluid injection processes, particularly injection of water or aqueous solutions, natural ionic equilibrium will be disturbed in the injection zone in the environment of wells. Clay minerals are also involved in said disturbance of the state of equilibrium. Generally, the less compatible the injected water is with the original ionic background of the system, i.e. with the clay minerals, the greater the disturbance to the natural ionic equilibrium. For this reason it is very important that prior to every single injection process both clay minerals and natural state of equilibrium of rock-liquid be carefully analyzed.